When installing a display device for medical purpose to be used as a monitor in, for example, an environment where a magnetic field generation device, such as an MRI (Magnetic Resonance Imaging system), is used, electronic parts that drive the display device might possibly cause malfunction due to changes of the surrounding magnetic field and perform operations that are not assumed. In particular, since an inductor included in a power supply circuit for driving the display device is directly affected by the surrounding magnetic field, the inductor may perform operations different from expected operations. This may cause the power supply circuit to malfunction. In addition, it can be considered that an integrated circuit (IC) is also affected by a magnetic field, and the integrated circuit may also perform operations different from expected operations.
In particular, a switching circuit is used for a voltage step-up circuit or a voltage step-down circuit as a component of a power supply circuit (DC/DC converter). The switching circuit is mainly constituted by an inductor, a field-effect transistor (FET), a rectifying diode, and a smoothing capacitor, and generates an output voltage having a voltage value different from an input voltage by converting the input voltage. Further, the DC/DC converter is provided with an overcurrent prevention circuit for preventing danger in electronic parts, such as heat and fire, when the output current becomes unusually large.
When a display device including a DC/DC converter having such a configuration is placed into a space where there is a strong magnetic field (referred to as “in a strong magnetic field”), the inductor is affected by the magnetic field to approach toward magnetic flux saturation. This causes increase of the current value. If the current value exceeds a threshold of the overcurrent prevention circuit, the DC/DC converter enters into a latch operation. If the latch operation is activated, the DC/DC converter halts the output. As a result, the display device halts and displays nothing. That is, when the display device is placed in a strong magnetic field, there is caused a problem that the display device stops suddenly as a result of changes of the surrounding magnetic field and there is needed an operation work to restore the stopped display device.
In order to avoid such a problem, for example, Patent Literature 1 identified below discloses a power conversion device including: an AC/DC converter for converting an AC voltage of an AC power source into a DC voltage; a smoothing capacitor for smoothing the DC voltage; a switching circuit which is constituted by a power transistor and converts the smoothed DC voltage into an AC voltage by performing an ON/OFF control of the power transistor; a PWM circuit which compares a command voltage with a carrier wave and outputs a control signal for performing the ON/OFF control of the power transistor of the switching circuit; and a comparator which determines whether or not there is magnetic flux saturation of a reactor for suppressing higher harmonic noise connected between the AC power source and the power conversion device and changes a switching frequency of the carrier wave when it is determined that there is magnetic flux saturation. This publication describes the following. In order to prevent the magnetic saturation of the reactor for suppressing higher harmonic noise, an inverter device that drives a motor with variable speed compares the magnetic flux detected by the magnetic flux detector with a pre-designated threshold level. If the magnetic flux exceeds that level, the inverter device decreases the oscillation frequency for controlling the power transistor of the power supply circuit in order to avoid a resonance frequency of an LC constituted by a reactor and a smoothing capacitor provided next to the reactor. After decreasing the oscillation frequency, the magnetic flux is detected again. If the magnetic flux is larger than a previous detected magnetic flux, the inverter device increases the oscillation frequency to change the oscillation frequency within a frequency domain where the reactor does not undergo magnetic saturation.
Incidentally, Patent Literature 2 identified below discloses a power control device for generating an electric power for driving an amplifier circuit that amplifies signals for transmission. The power control device includes: a generation means for generating a PWM signal using a maximum voltage value per unit time of a transmitting signal that should be outputted from the amplifier circuit according to the signal for transmission; a selection means; and a power generation means which includes a plurality of inductors including inductors having different inductance, for generating the electric power on the basis of the PWM signal by a chopper circuit that uses an inductor selected from among the plurality of inductors by the selection means. The selection means acquires a value reflecting the maximum voltage value, compares the value with a pre-set one or more thresholds, and selects an inductor to be used in the power generation means according to the comparison result. This publication describes providing two series of voltage step-up circuit lines in advance in the voltage step-up circuit, and changing the voltage step-up path (mainly an inductor) according to, for example, an output voltage value and load.
Incidentally, Patent Literature 3 identified below discloses a device including a reactor and a switching element, for use in a voltage converter that converts an input voltage into a predetermined output voltage by controlling the amount of current that passes the reactor with periodic switching control of the switching element. The device includes: a current detection section for sensing the amount of the current of the reactor; and a detection control section for sensing a change in capacitance of the reactor on the basis of a plurality of sensed values each obtained in the current detection section at a plurality of moments including a moment different from a center moment of an ON period or an OFF period of the switching element and one the basis of a reference value with respect to the sensed value when the capacity of the reactor has a predetermined value. This publication describes sensing the current when the reactor current increases as a result of, for example, deterioration of the reactor, and switching the switch OFF or reducing the SW period when there is a difference between the sensed electric current and that of a normal state (reference value).
Incidentally, Patent Literature 4 identified below discloses an actuator driving device for driving an electromagnetic actuator. The actuator driving device includes: a voltage step-up circuit for increasing a power supply voltage; and an increased voltage control means in downstream of the voltage step-up circuit. This publication describes monitoring the input voltage, and when the input voltage decreases, making the output voltage small by switching a feedback partial voltage resistance value of the voltage step-up section, and switching on and off the switch according to the determined result of the input voltage sensor.
Incidentally, Patent Literature 5 identified below discloses a chopper circuit including: two main reactors which are two separated body and constitute one reactor equivalently; a main switch including one pole connected to one end of the serially-connected main reactors and the other pole directly connected to one voltage terminal of a DC power supply; a serially-connected body of a snubber diode and a snubber capacitor, connected between both poles of the main switch; and an auxiliary switch connected between a junction point of the snubber diode and the snubber capacitor, and a junction point of the serially-connected two main reactors. The auxiliary switch makes the voltage of the snubber capacitor zero voltage, thereby, to make the voltage at the time of turning on the main switch zero voltage. This publication describes, with respect to ON/OFF timing of each switch, a soft switching operation that switches S2 ON slightly earlier than S1, and a regenerative operation that switches S2 ON simultaneous with S1 or later than S1.